The title "Monsters Vs. Aliens" promises retro fun beyond what any movie could deliver, though the actual film by that title tries hard enough.

We get lots of aliens — actually, one alien cloned a bunch of times — but only five monsters; those wanting to see the screen packed with an army of homegrown creatures battling extraterrestrials will leave disappointed. In the movie, Area 51 is a vast secret holding area for monsters captured over the decades; there must be more beasties in there than the few we meet. Maybe they’re saving some for the sequel.

The monsters we do get, though, are pretty amusing, all inspired by absurd creature features of the ‘50s and ‘60s. There’s Dr. Cockroach (voice by Hugh Laurie), a mad scientist who, like David Hedison in The Fly, emerges from an experiment with insectoid limbs and head. There’s B.O.B. (Seth Rogen), a brainless blob. There’s the Missing Link (Will Arnett), a tip of the fin to Creature from the Black Lagoon. There’s Insectosaurus, a massive mutated grub not unlike Mothra. Finally, there’s Susan (Reese Witherspoon), a bride-to-be who gets hit with a meteorite and turns into a fifty-foot woman. (I also appreciated the nod to The Amazing Colossal Man in a bit with a giant syringe.)

I don’t know whether the directors, Rob Letterman (Shark Tale) and Conrad Vernon (Shrek 2), or the platoon of credited writers grew up on monster and alien flicks. What I do know is that there’s a difference between a movie driven by obsession and a movie that kind of coasts on a neat premise, and Monsters Vs. Aliens is definitely the latter. I had a good enough time at it, but someone like Tim Burton (whose Mars Attacks is a goofy underrated classic) or Guillermo del Toro (who you can tell loves monsters so much it hurts him) would’ve brought the kind of ripe madness that helps a tribute like this endure. Monsters Vs. Aliens gets most of its fuel from the witty characterizations, which is good, but also from references that have nothing to do with monsters or aliens, like Dance Dance Revolution or the Beverly Hills Cop theme song, which are just jokes thrown in for contemporary parents and will date the movie.

A terrific sequence unfolds on a traffic-congested bridge, with terrified drivers caught between the heroic monsters and a giant one-eyed robot. It feels like a demo reel; nothing else really captures the scale or excitement a movie like this should pack, especially with today’s technology. Meanwhile, we spend too much time hearing about Susan’s fretful relationship with her selfish weatherman fiancé (Paul Rudd), who before Susan’s transformation thinks she’ll be thrilled that their honeymoon will be not in Paris but in ... Fresno. (I chortled at that, having spent a week in Fresno, where someone was murdered a block from where I was staying.) The hip voice cast, which includes Stephen Colbert voicing a president who looks like Bruce Campbell, is entertaining enough, though only Hugh Laurie as Dr. Cockroach really seems to understand what kind of movie he’s in — or what kind of movie he should be in.

"Monsters Vs. Aliens" is a fine rainy-Sunday diversion, but if you want real monster and alien fun, hit up Netflix and rent some of the classics that inspired it. When the evil alien proclaims “Destroy all monsters!” I nodded and thought “Yeah, I really need to watch 'Destroy All Monsters' again.”